The multiphase flow meter has gradually replaced the conventional testing separator owing to such advantages as low investment, low operational cost and unnecessary for separating oil well products. The multiphase flow meter may provide on-line and real-time flow rates and phase fractions of oil, gas and water and provide basic metering data for an oil company to understand actual production condition and capacity of each single well and perform effective reservoir management and production optimization management.
Among many multiphase flow metering technologies, the phase fraction measurement technology based on gamma-rays has become a dominant technology in multiphase metering. At present, the gamma measurement technology used in multiphase metering utilizes single energy or dual gamma-ray energies to acquire an average absorption coefficient of fluid with respect to gamma-rays. As the respective absorption coefficient of oil, gas and water with respect to gamma-rays of a certain energy is definite, in a two-phase flow, single gamma-ray energy is used to acquire an average absorption coefficient to obtain percentages of two components in a mixture (gas and liquid content or water and oil content); dual gamma-ray energies are used to acquire two average absorption coefficients to obtain percentages of three components in a mixture, i.e. phase fraction of a three-phase mixed fluid (gas content, oil content and water content).
At present, the multiphase metering technology based on gamma-rays is mainly confronted with the following limitations and challenges: (1) the measurement accuracy is not high enough. As gamma-ray measurement technology needs to satisfy such a premise as a narrow-beam condition, it is only possible to acquire the components of a mixture within a very small solid angle between a radioactive source and a probe, but nothing can be done about the portions beyond such detection interval. Such method by replacing a whole by a part allows the measurement result to have requirements for overall homogeneity of fluid, which thereby also relatively reduces the whole measurement accuracy. (2) The spatial symmetry is poor. As the gamma-ray narrow-beam in a multiphase flow meter is of a conical structure, the space density of the rays is very high when proximate to a radioactive source (a cone roof), i.e. the fluid is frequently detected, while the density of the rays becomes thinner when proximate to a receiving end (a cone bottom) of the gamma-ray. Thereby, even for a mixture of oil, gas and water of the same average density within a cone, if it has different space distributions in a cone, its measured average densities may also be different. (3) The required measurement time is very long. In order to reduce measurement errors brought by the aforementioned problems (1) and (2), the multiphase flow metering needs averaging many times of instantaneous measurement results so that the measurement time required for one data point is quite long. Such averaging depending on ergodic statistic solves the problem of asymmetry in measurement space, but its effect in reducing the whole non-homogeneity affect is not ideal. (4) Under an extreme flow condition, the metering accuracy of certain single-phase flow may be greatly reduced. At present, the multiphase metering technology bases on measurement of the total flow and the phase fraction and then obtain a flow rate of each single-phase through calculation; when certain single-phase is relatively less (the single-phase phase fraction is relatively lower), the metering accuracy of said phase is relatively poor. (5) It is impossible to obtain flowing behavior of a fluid flow pattern. The present gamma multiphase flow metering all can only provide average density information of a multiphase flow rather than position information of its specific space distribution. Thereby, it is impossible to provide space distribution information, i.e. imaging information, of oil, gas and water inside an oil pipeline.
One object of the present invention is to image a tomography of oil, gas and water inside an oil pipeline so as to obtain dynamic properties of a fluid flow pattern to form a real time video. This video information of fluid may provide richer metering information for petroleum industry such as to more effectively perform reservoir management and production optimization management. The position information of each single-phase fluid in a tomography image may also reduce the dependency on space distribution of flow patterns in the aforementioned absorption calculation as well as metering accuracy under an extreme flow condition, and thus may also greatly improve the measurement accuracy of phase fraction, which is also another object of the present invention. As the invention may improve the measurement accuracy, with respect to the original measurement accuracy, it is equivalent that the measurement time of each testing time is reduced.
The present imaging research on a multiphase flow uses technologies measuring medium features such as electrics/ERT, optics, ultrasound and magnetic resonance, such technologies all have their respective disadvantages and have not gained a successful application in the field of multiphase metering. The present invention uses gamma-ray imaging technology in nuclear medicine combined with relevant image processing technologies so as to effectuate tomography of a multiphase flow in an oil pipeline.